The present invention is directed generally to radiation curable compositions for use as protective surface coatings on metal substrates. Radiation curable compositions refer to any of those which cures on exposure to a beam of radiation including actinic, ultraviolet or ionizing radiation. The preferred source of radiation energy is ultraviolet (UV) radiation which generally provides useful wavelengths between about 1,600 A and 4,000 A.
One of the major problems experienced with radiation curable coatings applied to metal substrates has been the poor adhesion of cured film to metal surfaces. It has been postulated that the reason for the poor adhesion properties of such coatings is often the rapid and quick curing of the polymeric material accompanied with some film shrinkage such that bonding between the film and the substrate does not occur. Prior to radiation cure, heat cured coating materials showed generally acceptable adhesion properties to metal substrates. In curing the resinous coating composition of the present invention, any source supplying actinic, U.V., or ionizing radiation can be used, although for efficiency and economy, U.V. sources such as electric arc lamps, plasma arc torch (PARS) as described in U.S. Pat. No. 3,364,387 are preferred. Electron beam, when available, can be used also in which case the composition does not have to have a photosensitizer.
A radiation curable coating is suggested in U.S. Pat. No. 3,912,670 wherein a radiation curable oil containing a flow control additive and a reactive solvent-monomer contain acrylic acid. The flow control additive is a thermoplastic polymer having Newtonian flow characteristics and particularly containing no less than 5 to 20 weight percent hydroxyls. The thermoplastic additives must contain hydroxyls so as to provide hydrophilic units on the polymeric additive and impart wetting characteristics of the film to the metal substrate in accordance with well known hydrophilic coating technology for obtaining adhesion. Although the hydroxyl functionality apparently improves adhesion, the thermoplastic nature of the flow control additive causes less than desirable film properties such as poor mar resistance, poor solvent (MEK) resistance, and high viscosity of the coating.
It now has been found that UV and other radiation curable coating compositions cured by light energy and without heat obtain excellent adhesion to metal along with attendant solvent resistant and mar resistant surface coating films by providing a radiation curable composition particularly containing an adhesion promotor additive of a certain high T.sub.g polymeric material having a glass transition temperature (T.sub.g) above the ambient application temperature wherein the high T.sub.g polymer contains one ethylenically unsaturated group pendant on a long chain polymer. The high T.sub.g additive avoids the necessity of conventional hydrophilic or hydroxyls groups which tend to diminish cured film integrity properties and is believed to particularly achieve adhesion throughout the long chain polymer structure but substantially improve the film integrity properties by cross-linking the pendant ethylenic double bond with unsaturated double bonds in the ethylenically unsaturated binder polymer.